Tropical Cyclone Report (TCR): Hurricane Leslie
Tropical Cyclone Report
30 August-11 September 2012
Leslie was a long-lived tropical cyclone that meandered north of the Leeward Islands, reached hurricane intensity twice, and made landfall along the southeast coast of Newfoundland as a hurricane-force extratropical cyclone.
a. Synoptic History
Leslie developed from a vigorous tropical wave that moved off the coast of Africa on 26 August. The wave was convectively active, with the associated cloud pattern first showing signs of organization around midday 28 August, at which time a broad surface area of low pressure appears to have formed along the wave axis. Curved bands formed the next day as the system moved westward to the south of a mid-level ridge. Situated beneath an anticyclone, the low pressure area continued to become better organized, and it is estimated that the system became a tropical depression around 0600 UTC 30 August, centered approximately 1200 miles east of the Lesser Antilles. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively ( to be added in the final rendition). The best track positions and intensities are listed in Table 1 (to be added in the final rendition). Under light shear and very warm waters, the depression quickly became a tropical storm about 12 h later. Northerly shear increased over Leslie, which caused the center to become partially exposed along the northern edge of the convective cloud shield the next day. Leslie also moved very slowly during this time as a departing trough over the western Atlantic gave way to increased ridging north of the tropical storm. Despite the shear, Leslie steadily strengthened, and it reached an initial peak of 60 kt, just under hurricane strength, for a short time near 0000 UTC 1 September. At that time, Leslie was located about 425 miles east of the Leeward Islands. Leslie began to weaken thereafter as the shear increased dramatically, leaving the center well-removed from the shower activity. Leslie slowed down and moved slowly west-northwestward well to the northeast of the northern Leeward Islands.
A large burst of convection blossomed in association with the Leslie vortex late on 2 September, which expanded into 3 September. The cyclone appeared to become a little better organized during this time, although microwave data suggests that the low-level center was still straddling along the edge of the convective canopy -- west this time due to increasing westerly shear. The cloud pattern abruptly deteriorated after 0600 UTC 4 September. Leslie weakened during this time, likely due to the westerly shear mentioned above, as well as dry air entering the western portion of the large circulation as seen by arc clouds emanating westward away from the center on satellite animations (not shown). Convection began to ignite again just after 1800 UTC that day, but scatterometer ambiguities indicate that the low-level center had lost some definition relative to previous days, probably symptomatic of the westerly shear that had penetrated the inner core region. Synoptic scale steering in the region surrounding Leslie weakened further as broad upscale troughing began to develop over the eastern seaboard, and Leslie responded by becoming stationary on 5 September. The system also became a hurricane at 1200 UTC that day, still not moving. Leslie maintained hurricane strength for about a day, when it weakened back to a tropical storm. Since the upper tropospheric shear over the area appeared to be relatively light, it seems that the primary cause of weakening was cold water upwelling caused by the quasi-stationary motion of the system; analysis of areal sea surface temperatures in that area indicates that the hurricane cooled the surrounding waters by an average of 3C during the 72-hour period from 4 September through 7 September, with localized cooling of near 6C.
Convection diminished considerably shortly after Leslie lost hurricane status, and the cyclone weakened to an intensity of 45 kt by 1200 UTC 7 September. Leslie turned northwestward late on 7 September, and turned northward early on 8 September as the surrounding steering regime became a little more concentrated. As the former hurricane gradually increased its forward speed in the mid-latitude southerly flow associated with the east coast trough, it left its cold water wake. As this occurred, Leslie made several unsuccessful attempts at redeveloping an inner core, but it was never able to do so, likely because the outer wind maxima was expanding. In fact, surface and satellite data indicate that the circulation actually became larger during this time as the primary convection was focused in large bands well-removed from the center. Nevertheless, aircraft data during a flight into the system early on 8 September indicate that Leslie's winds had increased back to 50 kt. Leslie turned toward the north-northeast late the next day as it continued to accelerate, its center passing about 150 miles east of Bermuda around 1800 UTC that day. Probably due to baroclinic forcing from the approaching negatively-tilted upper trough, Leslie's winds continued to increase, and the cyclone is estimated to have regained hurricane status around 0600 UTC 10 September, centered about 600 miles south-southwest of Cape Race, Newfoundland.
AMSU vertical temperature profiles and surface observations indicate that a well-defined warm front was approaching Leslie from the west, and Leslie transitioned to an extratropical cyclone a few hours prior to landfall, which occurred along the southern coast of Newfoundland about 10 miles north of Placentia around 1215 UTC 11 September with an intensity of 65 kt, although given the highly baroclinic environment, most of the associated weather was well-removed from the center and reached the coast hours prior to the arrival of the center. The post-tropical remnants of Leslie were absorbed by a frontal zone over the Labrador Sea later that day.
b. Meteorological Statistics
Observations in Leslie (Figs 2 and 3, to be added in the final rendition) include the Dvorak satellite intensity intepretation technique used by the Tropical Analysis and Forecast Branch (TAFB), the Satellite Analysis Branch (SAB), and the Advanced Dvorak technique (ADT) used by the University of Wisconsin CIMSS. Several ASCAT passes were useful in determining the tropical storm force wind radii associated with Leslie, as well as for estimating the horizontal extent of its surface circulation. Indeed, the circulation appeared to have become considerably larger after Leslie pulled away from the Leeward Islands. Various microwave data were also used to track the center of Leslie. In addition, several drifting buoys owned and operated by Environment Canada were useful in ascertaining the intensity of Leslie when it made landfall.
The peak intensity of Leslie, estimated at 70 kt, is estimated to have occurred at 1800 UTC 5 September when the satellite signature appeared to be at its most impressive, and is based on a blend of the Dvorak numbers, which supported an intensity of about 65 kt from TAFB/SAB, and which supported an intensity closer to 75 kt by the UW-CIMSS ADT estimates. It should be noted that aerial reconnaissance was unavailable at the time of peak intensity, and the first flight into the system was not until 8 September, when Leslie had already weakened to a tropical storm.
Leslie's restrengthening to a hurricane again at 0600 UTC 10 September is difficult to pinpoint, since the satellite signature was not one of a typical hurricane. The rapidly cooling (but still sufficiently warm for tropical cyclogenesis/convective sustenance) sea surface temperatures along the track of the hurricane along with increasing southwesterly shear associated with the western Atlantic cold front, would definitely not argue for classical tropical intensification. However, AMSU microwave data still showed a solid warm core associated with Leslie at that point. Also, extrapolaton of data from a reconnaissance flight into the cyclone on 8 September suggest that the maximum surface winds associated with Leslie had increased to about 50 kt as the cyclone pulled away from its cold water wake it had generated north of the Leeward Islands. Since the convection actually became deeper -- albeit not concentrated near the center -- after departure of the aircraft, it is presumed that Leslie was eventually able to bring sustained hurricane force winds -- in small patches -- down to the surface, the vigor of the circulation and baroclinic forcing allowing these winds to persist even after the convection had dissipated. Finally, the acceleration of the hurricane as it approached Bermuda would argue for increasing winds, particularly in the eastern semicircle. Given the marginal environment over that portion of the Atlantic during 10 September, the previously alluded to upper-level trough, and the asymmetric cloud pattern and distribution of the wind field, and the lack of a well-defined inner core, it is presumed that a large portion of Leslie's intensification that day was related to baroclinic, rather than tropical processes. However, nature produces a wide spectrum of cyclones, tropical and extratropical, and without reconnaissance observations to contradict the aforementioned observations, Leslie is deemed to have reached hurricane strength near 0600 UTC 10 September, as it was pulling away from Bermuda.
Sustained tropical storm force winds occurred on Bermuda, where a sustained wind of 34 kt was reported on Saint David's on at 1631 UTC local time on 9 September; this occurred concurrently with a gust of 46 kt.
Leslie produced strong winds over portions of Atlantic Canada. St. John's, located on the southeastern tip of Newfoundland, reported a sustained wind of 51 kt with a gust to 71 kt around 1430 UTC local time on 11 September. Additionally, a sustained wind of 43 kt with a gust to 66 kt was recorded at Cape Race at 1203 UTC local time, also on 11 September.
Heavy rains also accompanied Leslie, with the Bermuda airport recording a storm maximum rainfall of 5.18 inches during the 48-hour period from 8 to 10 September. Leslie produced heavy rainfall over portions of Atlantic Canada, although its acceleration in the mid-latitude westerlies prevented a significant flood disaster. A storm maximum of 6.50 inches was observed at Shubenacadie in Nova Scotia.
Storm surge flooding was minimal since storm hit at low tide.
The lowest observed central pressure in Atlantic Canada was 973.1 mb at Sagona Island, which was noted at 1100 UTC local time 11 September.
Some rip currents and high waves were noted across portions of the eastern United States coast due to Leslie's large wind field. Rough surf also impacted portions of the northern Leeward Islands as the tropical cyclone passed by to the north.
c. Casualty and Damage Statistics
No fatalities have been attributed to Leslie, but the storm is estimated to have caused around $10 million in damage (2012 USD). Damage on Bermuda was minor and limited primarily to downed tree limbs and power lines, which caused some localized power outages. Leslie passed farther east of Bermuda than originally thought, which kept the island on the weaker side of the tropical cyclone.
Damage was more extensive across Atlantic Canada, where an extensive power outage was reported along the Avalon Peninsula in Newfoundland, where as much as 45,000 homes lost electricity for about six hours. In Pouch Cove, which is located on the Avalon Peninsula, Leslie destroyed several partially constructed homes in Pleasantville, which is a neighborhood located on the east side of St. John's on the far eastern tip of Newfoundland. In addition, some localized flooding occurred in the region, which caused some road and bridge closures.
d. Forecast and Warning Critique
Track forecasts associated with Leslie were fairly good, except for when the storm had begun to move and was approaching Bermuda. During that period, forecasts continuously called for Leslie to pass west and very near the island, based primarily on the GFS model, which was consistent on a track that brought Leslie to the west of Bermuda as opposed to east like actually occurred. It should be noted that there large discrepancies within the various computer model forecasts during that time. Leslie's smooth recurvature into the westerlies was correctly forecast, as was its slow motion and eventual trajectory into Atlantic Canada.
Intensity forecasts errors were considerably large, especially beyond 48 hours, where they exhibited a high bias. Several of the forecasts for Leslie called for the system to reach the threshold of Category 2/3 intensity as the cyclone made its closest approach to Bermuda. There were fears that Leslie would be a repeat of Hurricane Fabian in 2003, which directly struck the island at category 3 intensity. Leslie's intensity when it was meandering in the western Atlantic, and when it was approaching Newfoundland, was well predicted.
A tropical storm watch was issued for Bermuda at 2100 UTC 6 September, which was nearly four days before the arrival of tropical storm force winds. A tropical storm warning was issued for the island about 36 hours later. A hurricane watch was issued for sections of Atlantic Canada at 0600 UTC 10 September by Environment Canada.
A summary of the watches and warnings given with Leslie are indicated in Table 3 (to be added in the final rendition).
Visible satellite image of Leslie at peak intensity near 1800 UTC 5 September.